Abstract
Background:
The global antimicrobial resistance (AMR) crisis in poultry production has intensified the search for sustainable alternatives, specifically host-adapted probiotics. The Vietnamese CV Super M duck (Anas platyrhynchos domesticus) is an economically vital breed; however, its gastrointestinal tract (GIT) remains an underexplored reservoir for specialized microbial communities that may offer superior protective benefits compared to terrestrial poultry strains.

Aim:
This study aimed to isolate and characterize lactic acid bacteria (LAB) from the GIT of healthy CV Super M ducks and evaluate their in vitro probiotic potential and antimicrobial efficacy against prominent avian pathogens.

Methods:
With a sample size of 40 ducks, 60 LAB isolates were obtained through biochemical analysis. Neutralized cell-free supernatants (nCFSs) were used for screening to evaluate antimicrobial activity against Salmonella enterica, Escherichia coli, and Staphylococcus aureus. The most potent isolate, CV12, was evaluated for in vitro tolerance to simulated gastric acid (pH 2.0) and intestinal conditions (0.60% bile salt). Molecular characterization was performed using 16S rRNA gene sequencing.

Results:
Neutralized cell-free supernatants screening results demonstrated broad-spectrum antimicrobial activity: 37 isolates (61.67%), 33 isolates (55.00%), and 32 isolates (53.33%) inhibited S. enterica, E. coli, and S. aureus, respectively. Isolate CV12 is a highly promising strain, exhibiting excellent in vitro tolerance and high viability under gastric acid (4.45 ± 0.07 Log CFU/ml) and bile salt conditions (4.52 ± 0.14 Log CFU/ml). 16S rRNA sequencing identified CV12 as a member of the genus Companilactobacillus, exhibiting 98.42% similarity to Companilactobacillus farciminis. Notably, this identifies level falls below the common 98.7% species-identify threshold, indicating that CV12 may represent a novel species-level lineage specifically adapted to the GIT.

Conclusion:
Companilactobacillus sp. CV12 is a host-adapted strain with significant probiotic and antibacterial potential. This study provides the first evidence that the GIT of CV Super M ducks harbors unique, resilient microflora capable of serving as targeted probiotics. Further in vivo trials and whole-genome safety evaluations are required to confirm the commercial viability of this drug.

Key words: Anas platyrhynchos domesticus; Antimicrobial activity; Companilactobacillus sp.; Lactic acid bacteria (LAB); Probiotic.